The invention concerns an illumination system, particularly such a system for lithography, thus, for example, VUV and EUV-lithography with wavelengths equal to or less than 193 nm, which illuminates a field with an aspect ratio that is not equal to 1:1, wherein the illumination system comprises at least one light source, one field mirror or one field lens as well as optical components for transforming the light source into secondary light sources.
In order to be able to still further reduce the structural width of microelectronic devices, particularly to the submicron range, it is necessary to reduce the wavelengths of the light utilized for microlithography.
For example, lithography with weak x-ray radiation at wavelengths smaller than 193 nm, is conceivable, as has become known from U.S. Pat. No. 5,339,346.
In addition to illumination according to U.S. Pat. No. 5,339,346, which requires at least four mirror facets arranged symmetrically in pairs in respect to the source, illumination systems can be provided, which operate, for example, by means of plates with reflective raster elements for the homogeneous illumination of the annular field of an exposure objective. Such systems have the advantage that the field of an objective is illuminated homogeneously with as few reflections as possible, and also that an illumination of the pupil up to a specific filling ratio independent of the field is assured.
Reflective raster element plates for EUV-illumination systems have become known from U.S. Pat. No. 5,581,605.
The disclosures content of both of the above-named documents, U.S. Pat. No. 5,339,346 as well as U.S. Pat. No. 5,581,605 are incorporated in their entirety in the present application.
The following light sources are discussed at present as light sources for EUV-illumination systems:
laser plasma sources
pinch plasma sources
synchrotron radiation sources
In the case of laser plasma sources, an intensive laser beam is focussed onto a target (solid, gas jet, droplet). The target is heated so strongly by the excitation that a plasma is formed. This emits EUV-radiation.
Typical laser plasma sources have an angular characteristic like a sphere, that is the source radiates in each direction with nearly the same intensity, i.e., a radiation angle range of 4 xcfx80 as well as a diameter of 50 xcexcm to 200 xcexcm.
For pinch plasma sources, the plasma is produced by means of electrical excitation.
Pinch plasma sources can be described as volume radiators (=1.00 mm), which radiate in 4 xcfx80, whereby the beam characteristic is given by the mechanical source geometry.
In the case of synchrotron radiation sources, one can currently distinguish among three types of sources:
bending magnets
wigglers
undulators
In the case of bending magnet sources, the electrons are deflected by a bending magnet and photon radiation is emitted.
Wiggler sources comprise a so-called wiggler for deflecting an electron beam, and this wiggler comprises a multiple number of alternating polarized pairs of magnets arranged in rows. If an electron passes through a wiggler, then the electron is subjected to a periodic, vertical magnetic field; the electron correspondingly oscillates in the horizontal plane. Wigglers are also characterized by the fact that no coherency effects occur. The synchrotron radiation produced by means of a wiggler is similar to that of a bending magnet and radiates in a horizontal solid angle. In contrast to the bending magnet, it has a flux which is intensified by the number of poles of the wiggler.
There is no clear dividing line between wiggler sources and undulator sources.
In the case of undulator sources, the electrons in the undulator are subjected to a magnetic field with shorter periods and smaller magnetic field of the deflection poles than in the case of the wiggler, so that interference effects occur in the synchrotron radiation. The synchrotron radiation has a discontinuous spectrum based on the interference effects and emits both horizontally as well as vertically in a small solid-angle element; i.e., the radiation is highly directional.
Since the extension and angular spectrum of the currently discussed EUV-light sources are insufficient for filling or for illuminating field and aperture in the reticle plane of a lithography projection exposure system, the illumination systems presently discussed comprise at least one mirror or one lens with raster elements for producing a multiple number of secondary light sources, which are distributed uniformly in the diaphragm plane. Since the geometric form of the raster elements of the first mirror or of the first lens corresponds to the form of the illuminated field in the reticle plane, raster elements of the first mirror or of the first lens are preferably formed as rectangles, if the field to be illuminated is rectangular or represents a segment of an annular field. The optical effect of the raster elements of the first mirror or of the first lens, which are also designated below as field raster elements or field facets, is designed in such a way that images of the light source are formed in the diaphragm plane. Such light sources are so-called secondary light sources.
An EUV-illumination system, in which a number of secondary light sources are formed in a plane by means of two one-dimensional arrays, which are arranged perpendicular to each other, has been made known, for example, from U.S. Pat. No. 5,677,939. It is a disadvantage with this arrangement that two arrays of cylinder mirrors are necessary in order to illuminate a field with large aspect ratio and the exit pupil of the illumination system simultaneously.
In a second example, a system is shown in U.S. Pat. No. 5,677,939, in which critical Kxc3x6hler illumination is produced by means of a one-dimensional array of cylinder mirrors. It is disadvantageous in this case that the exit pupil of the illumination system is illuminated by single lines and thus is illuminated nonuniformly.
If the extent of the light source is small, for example, approximately like a point source, as in the case of an undulator source, then the extent of the secondary light sources formed by the field raster elements is also small. All light rays coming from the field raster elements are focused to the point-like secondary light sources. In this case an image of the field raster element is formed in each plane after the corresponding secondary light source wherein the imaging scale is given by the ratio of the secondary light source/reticle distance to the field raster element/secondary light source distance. The field raster elements are tilted such that the images of the field raster elements overlap in the reticle plane, at least in part.
The edge sharpness, i.e., the distance between the 0% point and the 100% point of the intensity distribution of the image of the field raster elements, is almost zero in the case of point-like light sources, i.e., the intensity decreases from 100% directly to 0% for ideal imaging.
In the case of extended light sources, the secondary light sources are also extended, so that the image of the field raster elements in the reticle plane is not sharp. The edge sharpness of the image of the field raster elements increases.
If one wishes to prevent an over-illumination of the pregiven width of the field to be illuminated, then this can be achieved by reducing the height of the field raster elements. Field raster elements designed in such a way have a high aspect ratio.
Field raster elements with high aspect ratio are also caused by the fact that the field to be illuminated has a large aspect ratio, for example, an x-y aspect ratio of 17.5:1. An aspect ratio is also known as a lateral magnification.
Field raster elements with a high aspect ratio, however, can be distributed only unfavorably on a field raster element plate and are expensive to manufacture.
The object of the invention is thus to create an illumination system, particularly for EUV-lithography, which has a simple structure and in which the disadvantages of the prior art can be avoided, and wherein the field to be illuminated has an aspect ratio #1:1.
According to the invention, the object is solved in that a portion of the optical components of the illumination system produce an anamorphotic effect. This leads to the fact that the secondary light sources are split into tangential and sagittal secondary line sources, wherein the lines sources are oriented perpendicularly.
The tangential secondary light sources are formed by means of a ray bundle in the y-z plane, and the sagittal secondary light sources are formed by a ray bundle running in the x-z plane. The direction of the larger dimension of the field to be illuminated is taken as the x-direction, and the direction of the smaller dimension of the field to be illuminated is taken as the y-direction in the following description, and the direction standing perpendicular to both the x-direction as well as the y-direction is denoted the z-direction. The y-direction is also the scanning direction in the case of a scanning lithography system, and the x-direction is the direction perpendicular to the scanning direction.
The optical components for the transformation of the light source into secondary light sources comprise at least one mirror or one lens, which is or which are organized into raster elements, wherein the raster elements have a two-dimensional arrangement. By this feature, a very regular illumination of the exit pupil can be achieved.
For the illumination of the mirror or the plate with raster elements, a collector unit is used, which is adapted to the respective source.
The two-dimensionally arranged raster elements on the first mirror or lens are preferably arranged in individual rows adjacent to one another, whereby each row comprises several raster elements lying next to one another. The individual rows are in turn displaced towards one another, which leads to a more uniform illumination of the exit pupil.
One embodiment of the invention provides that the raster elements of the mirror or of the lens are formed for producing sagittal and tangential secondary light sources in such a way that each raster element produces an anamorphotic effect, for example, by forming the raster elements of the first mirror as cylinders or toroids.
In a particular configuration of the invention, it is provided that the collector unit has an anamorphotic effect, and thus the light source is imaged in a secondary light source, which is split into tangential and a sagittal light sources.
The field that is to be illuminated comprises a rectangular shape or an annular segment, so that the raster elements are advantageously configured as rectangles. Since the raster elements are imaged in the field plane, they are also designated as field raster elements.
In the case of an embodiment with very large aspect ratios of the field to be illuminated, for example, an x-y aspect ratio of 17.5:1, 13:1 or 2:1, it is advantageously provided that the aspect ratio of the cylindrical or toroidal field raster elements is smaller. In the case of an x-y aspect ratio of the field to be illuminated of 17.5:1 or 13:1, the aspect ratio of the field raster elements can be, for example, 4:1; in the case of an x-y aspect ratio of the field to be illuminated of 2:1, for example, that of the field raster elements can be 1.5:1.
Depending on the field mirror(s) or field lens(es) used in the illumination system, it is advantageous to arrange the field raster elements on the first mirror or lens in such a way that the images of the secondary light sources, which are produced by the field mirrors or field lens(es) and which are designated herein below as xe2x80x9ctertiary light sourcesxe2x80x9d, are distributed in an extensively uniform manner in the exit pupil of the illumination system. The tertiary light sources are sometimes also called xe2x80x9cpartial pupilsxe2x80x9d.
For this purpose, the rastering of the tertiary light sources in the exit pupil of the illumination system, i.e., the positions of the tertiary light sources in the exit pupil, is first provided in advance. Then, the position of the field raster elements on the field raster element plate is determined by tracing the principal rays of the tertiary light sources backwards through the field lens or field lenses up to the plane, which is defined by the field raster element plate. The direction of the principal rays is given in advance, so that they intersect the center of the reticle plane. Their intersection points on the field raster element plate determine the positions of the field raster elements. For this type of determination of the positions of the field raster elements, the boundary condition must be maintained that the distance between the tertiary light sources in the pregiven rastering is selected such that the field raster elements do not overlap.
Based on optical imaging errors of the field lens imaging, e.g., distortion, the field raster elements may be asymmetrically arranged on the field raster element plate.
In order to compensate for the intensity tilt in the pupil illumination caused by the asymmetric arrangement of the raster elements, it is advantageously provided to adapt the reflectivities of the raster elements in an appropriate manner.
It is also possible to correspondingly adapt the collector unit.
In order to obtain a sharp image of the field raster elements in the reticle plane in systems with extended light sources, one can advantageously provide a second mirror or a lens with raster elements, wherein the raster elements of the second mirror or of the second lens are arranged at the position of the secondary light sources. The raster elements of the second mirror or of the second lens are called pupil raster elements.
In the case of systems with two mirrors with raster elements, the form of the raster elements of the second mirror, i.e., the pupil raster elements, is adapted in an advantageous manner to the form of the secondary light sources and thus differs from the form of the first raster elements, i.e., the field raster elements. For example, the pupil raster elements can be of round shape in case the light source is round in shape. Whether or not pupil raster elements are necessary depends on the design of the field raster elements and the extent and the angle distribution of the light source. The product of the spatial and angular extent of the light source at the source point of view view, or the product of the field extent and aperture at the field point of view, is designated as the Etendu or Lagrange optical invariant (light conductance (LC)). The Etendu can be defined one-dimensionally or two-dimensionally. The tangential Etendu considers only rays that run in the y-z plane. In this case the Etendu is given by the product of the y-field height and the aperture in the y-z-plane. The two-dimensional Etendu considers all rays. In this case the Etendu is given by the product of the field area and the square of the aperture.
Depending on the type of light source, the following cases can be distinguished:
1. Tangential Etendu of the source less than  less than tangential Etendu of the field to be illuminated, i.e., edge sharpness less than  less than y-field dimension. An illumination system without pupil raster elements is possible.
2. Tangential Etendu of the source=tangential Etendu of the field to be illuminated, i.e., edge sharpness=y-field dimension. A design without pupil raster elements is possible, if components with anamorphotic effect are used. The illumination should be made critical in the y-direction and it should be made a Kxc3x6hler illumination in the x-direction. In order to image the x-direction with sharp edges, cylindrical or toroidal raster elements can be used as pupil raster elements at the location of the sagittal light sources in a further embodiment of the invention.
3. Two-dimensional Etendu of the sourcexe2x89xa6two-dimensional Etendu of the field to be illuminated and tangential Etendu of the source greater than tangential Etendu of the field. In this case, it is necessary to provide pupil raster elements for the correct illumination of the field.
For application of the illumination system in a scanning lithography system, it is not absolutely necessary to image the narrow side of a field raster element, i.e., the y-direction, so that the edges are sharp. It is only important that the illuminated field lies within the object field defined by the subsequent projection objective. Typical projection objectives for EUV-Lithography have ring-shaped or annular object fields.
As long as this can be achieved with expanded light sources by the reduction of the width of the field raster elements or with the use of field raster elements with anamorphotic effect (up to the critical illumination of the narrow field side), it is not necessary to provide pupil raster elements for the imaging of the y-direction of the field raster elements.
However, if in critical illumination the illumination of the field exceeds the maximum possible field width in the y-direction, then pupil raster elements are necessary for the y-direction.
Perpendicular to the scanning direction, i.e., in the x-direction, the illuminated field should have sharp edges, since otherwise, the illuminated field perpendicular to the scanning direction, i.e., in the x-direction, would have to be limited by means of additional masking devices, which would lead to a loss of light due to the vignetting of the illuminated field. In the case of extended light sources, pupil raster elements can thus be advantageously provided for the x-direction.
What type of pupil raster elements are used depends on which requirements are established for field illumination.
If the edge sharpness of the images of the field raster elements need to be sharp only in the x-direction of the illuminated field, in the reticle plane, cylindrical pupil raster elements can be applied at the location of the sagittal secondary light sources. The cylinders are thus oriented in the direction of the sagittal secondary line light sources and image the long side of the field raster elements in the reticle plane.
If the imaging must also be influenced in the y-direction, then toroidal pupil raster elements are used. They image the long side of the field raster elements in the reticle plane and limit the illumination in the y-direction to the permissible range. They are arranged either at the location of the sagittal secondary light sources or between sagittal and tangential light sources. In the case of an arrangement between sagittal and tangential light sources, imaging of the field raster elements and the raster element dimensions must be appropriately adapted.
Field and pupil raster elements are arranged and aligned such that a light path is produced between each field and each pupil raster element; the superimposition of the images of the field raster elements is achieved in the reticle plane and the tertiary light sources are distributed as uniformly as possible in the exit pupil of the illumination system. With respect to the method for arranging the field and pupil raster elements, reference is made to U.S. Pat. No. 6,198,793, filed on May 4, 1999 with the title xe2x80x9cIllumination system, particularly for EUV-lithographyxe2x80x9d, of the Applicant, whose disclosure content is incorporated to the fall extent in the present application.
Advantageously, the field mirrors or the field lens(es) of the illumination system according to the invention are formed in such a way that the diaphragm plane is imaged in the exit pupil of the illumination system.
By splitting the secondary light sources into tangential and sagittal secondary light sources according to the invention, the following advantages can be obtained; of course they may also be combined:
1. Reduction of the raster element aspect ratio by optical components with anamorphotic effect.
In the case of a point-like light source, the field raster elements can be imaged in the image plane with a high edge sharpness, even with a raster element aspect ratio that is reduced in comparison to the field aspect ratio, whereby the anamorphotic effect must be designed correspondingly.
2. Illumination of the field in the case of expanded light sources.
In the case of expanded light sources, the edge sharpness, which is produced without pupil raster elements, can be taken into account in such a way that the tangential light source is shifted in the direction of the image plane. This is possible by means of a reduction of the y-refractive power of the field raster element with anamorphotic effect. In this manner, the y-width of the image of the field raster element is reduced. The critical illumination in the y-direction at which the tangential light source is situated in the reticle plane represents the limit of this shift. The source is then imaged in the y-direction in the image plane. The y-dimension of the image of the light source should in this case not exceed the y-width of the illuminated field. Otherwise, the illuminated field would have to be masked or pupil raster elements would have to be used.